1. Field of the Invention
This invention relates generally to electro-acoustical drivers and loudspeakers employing electro-acoustical drivers. More particularly, the invention relates to improved configurations for compression drivers and phasing plugs utilized in compression drivers.
2. Related Art
An electro-acoustical transducer or driver is utilized as a component in a loudspeaker system to convert electrical signals into acoustical signals. The driver includes mechanical, electromechanical, and magnetic elements to effect this conversion. For example, the electrical signals may be directed through a voice coil that is attached to a flexible diaphragm and positioned in an air gap. The voice coil is immersed in a radially oriented magnetic field provided by a permanent magnet and steel elements of a magnet assembly. Due to the Lorenz force affecting the conductor of current positioned in the permanent magnetic field, the alternating current corresponding to electrical signals conveying audio signals actuates the voice coil to reciprocate back and forth in the air gap and, correspondingly, move the diaphragm to which the coil is attached. The diaphragm may be suspended by one or more supporting elements (e.g., a surround, spider, or the like) such that at least a portion of the diaphragm is permitted to move. Accordingly, the reciprocating voice coil actuates the diaphragm to likewise reciprocate and, consequently, produce acoustic signals that propagate as sound waves through a suitable fluid medium such as air. Sound pressure differences in the fluid medium associated with these waves are interpreted by a listener as sound. The sound waves may be characterized by their instantaneous spectrum and level.
The driver at its output side may be coupled to an acoustic waveguide, which is a structure that encloses the volume of medium into which sound waves are first received from the driver. The waveguide may be designed to increase the efficiency of the driver and control the directivity of the propagating sound waves. The waveguide typically includes one open end coupled to the driver, and another open end or mouth downstream from the driver-side end. Sound waves produced by the driver propagate through the waveguide and are dispersed from the mouth to a listening area. The waveguide may be structured as a horn or other flared structure such that the interior defined by the waveguide expands or increases from the driver-side end to the mouth.
Electro-acoustical transducers or drivers may be characterized into two broad categories: direct-radiating types and compression types. A direct-radiating transducer produces sound waves and radiates these sound waves directly into open air (i.e., the environment ambient to the loudspeaker), whereas a compression driver first moves air in a radial direction in a high-pressure region, or compression chamber, and then produces sound waves that propagate in an axial direction to the typically much lower-pressure open-air environment. The compression chamber is open to a structure commonly referred as a phasing plug that works as a connector between the compression chamber and the horn. The area of the exit of the compression chamber (i.e., the entrance to the phasing plug) is smaller than the effective area of the diaphragm. This provides increased efficiency as compared to a direct-radiating loudspeaker. In a direct-radiating loudspeaker, the mechanical output impedance of the vibrating diaphragm is significantly higher than the loading impedance of the open air (the radiation impedance). This results in a mismatch between the “generator” (diaphragm) and the “load” (open air radiation impedance). In a compression driver, the loading impedance (entrance to the phasing plug) is significantly higher than the open air radiation impedance. This produces much better matching between the “generator” and the “load” and increases the efficiency of the compression driver as a transducer. Typically, it is considered ideal to attain 50% driver efficiency when the mechanical output impedance of the vibrating diaphragm is equal to the mechanical loading impedance of the phasing plug with the horn connected to it.
As noted, a compression driver utilizes a compression chamber on the output side of the diaphragm to generate relatively higher-pressure sound energy prior to radiating the sound waves from the loudspeaker. Typically, a phasing plug is interposed between the diaphragm and the waveguide or horn portion of the loudspeaker, and is spaced from the diaphragm by a small distance (typically a fraction of a millimeter). Accordingly, the compression chamber is bounded on one side by the diaphragm and on the other side by the phasing plug. The phasing plug is typically perforated in some fashion. That is, the phasing plug includes apertures (i.e., passages or channels) that extend between the compression chamber and the waveguide or horn portion of the loudspeaker to provide acoustic pathways from the compression chamber to the waveguide. The cross-sectional area of the apertures is small in comparison to the effective area of the diaphragm, thereby providing air compression and increased sound pressure in the compression chamber.
The compression driver, characterized by having a phasing plug and a compression chamber, may provide a number of advantages if properly designed. These advantages may include increasing the efficiency with which the mechanical energy associated with the moving diaphragm is converted into acoustic energy. Decreasing the parasitic compliance of air in the compression chamber prevents undesired attenuation of high-frequency acoustic signals. Properly positioning the apertures in the phasing plug and sizing the lengths of the associated passages may result in delivering sound energy in phase from all parts of the diaphragm, suppressing or canceling high-frequency standing waves in the compression chamber, and reducing or eliminating undesired interfering cancellations in the propagating sound waves. Particularly for high frequencies, compression drivers may be considered to be superior to direct-radiating drivers for generating high sound-pressure levels.
The diaphragm of a compression driver may have an annular shape and be coaxially disposed about central structures of the phasing plug. An annular diaphragm may have various configurations. As examples, the annular diaphragm may have a V-shaped cross-section (FIG. 27), an M-shaped cross-section (FIG. 28), a dual roll cross-section (FIG. 29), or various combinations of the foregoing as well as other shapes. Different shapes of annular diaphragms have their own advantages and drawbacks. As examples, the V-shaped diaphragm has the lowest resonance frequency (in comparison to other diaphragms having similar voice coils) but its flat suspension is the most nonlinear. The suspension of the V-shaped diaphragm has the shape of internal and external flat rings, which is the softest configuration but has limited displacement capability, i.e., the stiffness of the V-shaped diaphragm rapidly increases with displacement. In comparison to other diaphragms having comparable attributes (e.g., similar inside diameter, voice coil diameter, thickness of diaphragm, and material composition of diaphragms, the M-shaped diaphragm and the dual roll diaphragm have higher resonance frequency (stiffer suspensions) but their suspensions are significantly more linear because of their geometry. The application of an annular diaphragm of a particular shape depends on the requirements of the desired frequency range, the linearity of displacement, and the shape of the frequency response.
Annular diaphragms may be fabricated out of different materials. For example V-shaped diaphragms made of aluminum foil have been manufactured since the early 1950s for high-frequency compression drivers. More recently, compression drivers based on annular diaphragms are typically made of thermoformed polymer films. The capability of the driver to efficiently reproduce high frequency signals depends predominantly on the diaphragm's moving mass and on its high frequency breakups (i.e. partial resonances). At high frequency range the diaphragm does not vibrate as a solid shell, but rather its parts vibrate with different amplitudes and phases. At the resonances (breakups) the diaphragm's overall surface exhibits an increase of displacement and, velocity, and therefore the upper part of the frequency range is increased as well. Due to the high internal damping of polymer films the frequency response of plastic diaphragms is typically much smoother than that of the diaphragms made of aluminum or titanium. There are several factors that limit high frequency signal, including the moving mass of the diaphragm assembly and the volume of the compression chamber. The higher the moving mass, the lower is the high-frequency roll-off (the frequency where the response starts to decrease). The larger the volume of the compression chamber, the lower is the roll-off of the frequency response. Acoustical compliance of air in the compression chamber acts as a low-pass filter, and a larger height of the compression chamber causes a higher compliance of the “air spring”, and correspondingly, attenuation of high-frequency signals.
Extension of high frequency response could be obtained by decreasing the moving mass of the diaphragm assembly. However, this would require a smaller diaphragm and a smaller voice coil, which implies a smaller power handling capability. Attempts have been made to avoid this problem by manifolding compression drivers to make them work to a single acoustical load. In one example, several drivers have been mounted to the input ends of a Y-shaped or double Y-shaped tube, with a horn mounted to the single output end of the tube. In another example, several drivers have been stacked into a linear array, with circuitry provided on the input side of each driver to customize the individual frequency and directivity responses of the drivers. In another example, multiple drivers have been symmetrically mounted on opposing sides of a single horn structure, with the higher-frequency drivers being located behind the lower-frequency drivers relative to the mouth of the single horn. In another example, two compression drivers are arranged such that their respective diaphragms axially oppose each other and are coaxial with a central sound output bore. Each driver includes rotationally symmetric radial slots, all of equal length, across their respective compression chambers. The radial slots lead to radial channels that in turn lead to the central sound output bore. The radial slots of the one driver are interleaved with the radial slots of the other driver. That is, the circumferential positions of the radial slots of the one driver alternate with the circumferential positions of the radial slots of the other driver. None of these past approaches is considered to provide the performance criteria currently sought for compression drivers. For instance, the use of equal-length radial slots is disadvantageous in that they may fail to suppress circumferential resonances in the compression chamber, which may degrade the desired frequency response.
Accordingly, there exists an ongoing need for improved designs for compression drivers so as to more fully attain their advantages such as high-frequency efficiency, while ameliorating their disadvantages such as detrimental acoustical non-linear effects, irregularity of frequency response, and limited frequency range.